This artistic rendering shows the distant view from a theoretical ‘Planet Nine’ or ‘Planet X’ back towards the sun. The planet is thought to be gaseous, but smaller than Uranus and Neptune. Hypothetical lightning lights up the night side.Caltech/R. Hurt (IPAC)

The history of astronomy has been a history of receding horizons. The invention of the telescope took us beyond our naked-eye capabilities, to millions (and later billions) of stars within our own Milky Way. The application of photography and multi-wavelength astronomy to telescopes brought us beyond our own galaxy, to the distant “island Universes” populating all the space we can access. Yet, for all we know about the distant Universe, there still may be undiscovered worlds in our own Solar System. Why is that? Joseph Cummens wants to know, asking:

If scientists can use telescopes to hunt planets, galaxies, exoplanets, etc., then why can’t we scan our solar system for the elusive Planet X or other celestial bodies within our home system?

As far as we’ve peered into the Universe, we still have a long way to go, even in our own backyard.

Based on their orbital parameters, most of the objects from beyond Neptune fall into some well-known categories like the Kuiper belt or the scattered disk. Detached objects are rare, with Sedna perhaps the single most exceptional object of all for both its size and its orbital parameters. Outside of Neptune, but still in the Kuiper belt, are the objects that are the earliest, most pristine remnants from the period of planet-formation in our Solar System. Note that most of these objects are still undiscovered.Wikimedia Commons user Eurocommuter

There’s a key word you need to understand that puts the entire question into perspective: magnitude. From an astronomical perspective, every object has an intrinsic brightness to it, defined by the amount of light it gives off. For an object like our Sun, this is due to its own luminance, since the Sun creates its own energy and emits it in all directions. For an object like our Moon, this is due to its reflected luminance, since it only reflects the light from other objects. The Moon has no self-luminance of its own.

If you look at the Moon during its crescent phase, you can actually make out the signal from the lunar surface that isn’t illuminated by the Sun. This isn’t some trick of the Moon’s atmosphere (since it has virtually none), but rather is due to Earthshine: sunlight reflected off of the Earth and onto the Moon.

The crescent Moon, when it appears in the sky, has a crescent shape because that’s the portion illuminated by the Sun. The remainder of the Moon is still visible, however, due to the phenomenon of Earthshine, where sunlight reflected by the Earth lands on the Moon, and gets reflected back to Earth again.National Railway Museum/SSPL/Getty Images

The difference in brightness between these examples showcases how extreme the difference between reflected luminance and self-luminance are.

But there’s another thing that’s exemplified by the extreme brightness differences between the Sun and Moon, and the Moon and everything else in the night sky. The Moon has no right to appear brighter than every star, planet, or galaxy in the sky based on its own pitiful magnitude. Intrinsically, the Moon is the faintest object visible with the naked eye from anywhere on Earth. Yet it appears brighter than everything except the Sun!

The reason for this is that the Moon is so close, and that intrinsic brightness isn’t the same as observed — or apparent — brightness.

The way that sunlight spreads out as a function of distance means that the farther away from a power source you are, the energy that you intercept drops off as one over the distance squared.Wikimedia Commons user Borb

The farther away an object is, the less bright it appears. But this isn’t just some general rule we apply, there’s a quantitative relationship that allows us to determine how bright-or-faint an object appears based on its distance. Put simply, brightness falls off as the inverse of the distance squared, or b ~ 1/r2.

Place an object twice as far away, and it will appear one-fourth as bright. Place it ten times as distant, and it appears just one-hundredth as bright. And place it a thousand times as far from you as it started, and it will appear just one-millionth as bright as it was initially.

For any object that emits its own light, these two factors determine an object’s apparent brightness: the intrinsic brightness and the distance it is from the observer.

Reflecting telescopes surpassed refractors long ago, as the size you can build a mirror greatly surpasses the size to which you can build a similar-quality lens. Even if we took all the telescopes on Earth and dedicated them to trying to discover additional worlds in the Solar System, we wouldn’t catch them all.The Observatories of the Carnegie Institution for Science Collection at the Huntington Library, San Marino, Calif.

These two factors are, arguably, the two biggest ones to consider when we determine what type of telescope to build. Want to see something fainter? You’ll need to collect more light, which either means building a bigger telescope or observing the same portion of the sky for longer.

If money and engineering were no consideration, you’d opt for the bigger telescope every time. Build your telescope twice as large, and you not only gather four times as much light, but you double your resolution. To gather four times as much light by observing longer, you need to spend four times the amount of time, and gain no such advantage in resolution.

The biggest telescopes we have are capable of viewing objects to the greatest resolution possible, and resolving their details in the shortest possible time.

This diagram shows the novel 5-mirror optical system of ESO’s Extremely Large Telescope (ELT). Before reaching the science instruments the light is first reflected from the telescope’s giant concave 39-metre segmented primary mirror (M1), it then bounces off two further 4-metre-class mirrors, one convex (M2) and one concave (M3). The final two mirrors (M4 and M5) form a built-in adaptive optics system to allow extremely sharp images to be formed at the final focal plane. This telescope will have more light-gathering power than any telescope in history.ESO

There’s also the consideration of field-of-view. What’s your goal? Is it to see the faintest object possible? Or is it to view the greatest possible amount of the Universe?

There’s a trade-off to make. Your telescope can gather a certain amount of light, and it can either do that by viewing a small region to great precision, or a large region to lesser precision. Just as a microscope can double its magnification by halving the diameter of its field-of-view, a telescope can see deeper into a region of the Universe by narrowing its field-of-view.

Different telescopes are optimized for different purposes. The trade-off is severe, however. If we want to go as deep as possible, we can only do it in one small region of the sky.

Various long-exposure campaigns, like the Hubble eXtreme Deep Field (XDF) shown here, have revealed thousands of galaxies in a volume of the Universe that represents a fraction of a millionth of the sky. But even with all the power of Hubble, and all the magnification of gravitational lensing, there are still galaxies out there beyond what we are capable of seeing.NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

This is the Hubble eXtreme Deep Field. A tiny region of space was imaged, in a variety of wavelengths, for a total of 23 days. The amount of information that was revealed is breathtaking: we found 5,500 galaxies in this small patch of sky. The faintest objects in this patch are literally a factor of 10,000,000,000 (ten billion) times fainter than what you can see at the limit of your naked eye.

Due to its large-diameter mirror, its observations at a variety of wavelengths, its location in space, as well as its high magnification and small field-of-view, Hubble can reveal the faintest galaxies ever discovered. But there’s a cost: this image, which took 23 days of data to create, spans only 1/32,000,000th of the sky.

This compressed view of the entire sky visible from Hawai’i by the Pan-STARRS1 Observatory is the result of half a million exposures, each about 45 seconds in length. A survey as wide-field as Pan-STARRS could discover tens of thousands of Kuiper Belt objects, but it would need to see fainter objects than Pan-STARRS is capable of seeing.Danny Farrow, Pan-STARRS1 Science Consortium and Max Planck Institute for Extraterrestrial Physics

On the other hand, you can take a view like this. This was created with the Pan-STARRS telescope, which views the entire visible sky multiple times every night from its location here on Earth. It’s comparable in size to the Hubble Space Telescope, but it’s optimized for wide-field imaging, choosing to value sky-coverage over magnification.

As a result, it can reveal objects located practically anywhere on the sky; only the extreme south pole region is cut off due to the telescope’s location in the northern hemisphere. Pan-STARRS, which stands for Panoramic Survey Telescope and Rapid Response System, grabs some 75% of the sky, and is great for detecting changes between points of light. It can find comets, asteroids, Kuiper belt objects and more like no other. But it can only find objects that are thousands of times brighter than the faintest ones Hubble can detect.

Although Sedna was discovered way back in 2003, only one other object, 2012 VP113 (shown here), has been discovered that is classified as a Sednoid, and that possibly originates from the inner Oort cloud. Some people prefer the Planet Nine hypothesis, but that is a challenge for Sedna.Scott S. Sheppard/Carnegie Institution for Science

As much as we’d like to, we cannot simply survey the entire outer Solar System at the required magnitude to discover everything that’s out there. A super-deep, super-faint, all-sky survey will likely never be a possibility due to technological limitations; we can go faint-and-narrow or bright-and-wide, but not both, simultaneously.

There’s also one more limiting factor that goes way back to the beginning: these objects are only reflecting sunlight. If you look to the outer Solar System at two identical objects, but one’s twice as distant as the other, it’s actually only one-sixteenth as bright. This is because by time the sunlight hits the farther object, it’s only one-quarter as bright, but then that reflected light has to travel double the distance back to our eyes, making the overall apparent brightness fall off as b ~ 1/r4. Even if we had a Jupiter-sized world located in the Oort Cloud, we wouldn’t have found it yet.

Far beyond the Sun and the planets in our Solar System, the Kuiper belt exists. Yet even beyond that, there are a slew of other objects with often bizarre and confusing orbital properties out there. We hope, soon, to discover the correct explanation for why they are the way they are.Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

We have plenty of telescopes capable of seeing incredibly faint objects, but we need to know where to point them. We have plenty of telescopes capable of surveying huge areas of the sky, but they can only see the brighter objects; faint ones are out of reach. And for objects in our own Solar System, because they reflect sunlight rather than emit their own, self-generated light, they cannot be seen by any modern telescope if they’re located beyond a certain distance.

As with all things, the scanning we can do is powerful, interesting, and educational. It has revealed thousands upon thousands of objects within our own Solar System, from planets to moons to asteroids to Kuiper belt objects and more. But as telescope technology and sky coverage improve, we only see smaller, fainter, and more distant objects. We push the limits, but we never remove them. The science of astronomy is a story of receding horizons. But no matter how deep we go, there will always be a limit to what we can observe.